J.W. Davis

Isotopic effects in hydrocarbon formation due to low-energy H+/D+ impact on graphite

Recent developments with gaseous divertors in tokamaks have led to prospects of less energetic ion bombardment (10s of eV) of material surfaces in the divertor region. The present experiments were undertaken with the objective of studying hydrocarbon formation due to H+/D+ impact on graphite for energies extending down to the 10 eV range. Mass spectrometry in the residual gas was used to measure the hydrocarbon formation rates as a function of pyrolytic graphite temperature (300–1000 K) and ion energy (10–200 eV/H or D) for mass-analysed H3+ and D2+ beams (1018 H+(D+)/m2 s) providing a unique opportunity to also investigate isotopic effects. The results indicate that, as the ion impact energy is reduced, a reduction in the maximum chemical yield (Ym) is observed and the broadening of the temperature dependence profile for hydrocarbon formation leads to significant erosion for low-energy impact at room temperature. The room-temperature methane and total chemical yields display maxima at about 50 eV and decrease as the ion energy is further reduced. Where kinetic effects are expected to affect the erosion process, viz., low energy ( 50 eV)/near Tm, an isotopic yield ratio, YD/YH of about 1.5–2 was measured. Under all other conditions studied, this ratio was near unity. Furthermore, it is evident that the dominating erosion mechanisms at low energies (<100 eV) differ from those occurring for higher energy impact.

Deuterium retention in tungsten for fusion use

The retention of deuterium in polycrystalline W foils has been measured as a function of ion fluence and implantation temperature. At temperatures in the range 350-550 K, retention levels were found to be above the 300 K value. This retention enhancement is attributed to an increase in the D diffusion coefficient, which allows a greater diffusion depth. Furthermore, while retention at room temperature saturates as a function of fluence, at 500 K no saturation is observed. D( 3 He, 4 He)p nuclear reaction analysis measurements show that at room temperature most of the D is trapped in the near-surface region of the specimen, but significantly beyond the implantation range. At higher temperatures, much lower levels are observed in the near-surface, and diffusion through to the back of thin specimens is observed: the front and back surface D concentrations are similar. While the D retention in a proposed W alloy for ITER applications (W-1%La 2 O 3 ) is similar to that measured in the pure W foils over most of the range of the experiments, two important differences are noted: a trend to saturation of the amount retained is observed at fluences >10 24 D/m 2 and 500 K, and the D concentration at the back of the W-1%La 2 O 3 alloy is about 1% of that at the implanted surface.

Two‐region model for hydrogen trapping in and release from graphite

A new model has been developed for hydrogen retention and trapping in and release from graphite. Two different regions in the graphite with different hydrogen transport and trapping behaviors are distinguished, the bulk region within, and the surface region on graphite crystallites. The model incorporates new experimental results related to atom diffusion and recombination on inner surfaces. Recombination is explained from a fundamental viewpoint by linking it to diffusion using a classical expression. The model is applied to a number of reemission and thermal desorption experiments, in particular, the reemission of hydrogen atoms during irradiation with energetic hydrogen ions and the formation of HD during irradiation with H+ and D+ or during thermal desorption of graphite that was preimplanted with H+ and D+ ions with different energies.

Hydrocarbon formation due to combined H + ion and H0 atom impact on pyrolytic graphite

Graphite, which is a favoured material for first wall use in fusion devices, is subject to chemical erosion under hydrogenic impact. Until recently, most published results on hydrogen-induced chemical erosion of graphite have concentrated only on the production of methane. For incident H+ energies ≳ 300 eV/H+, methane is in fact the dominant reaction product. For low energy (< 300 eV) H+ ion impact on pyrolytic graphite, however, the relative formation rates of heavier hydrocarbons (C2Hx, C3Hx) become more important. For thermal H0 atom (sub-eV) impact, the formation of heavier hydrocarbons dominates the total carbon erosion process. For combined H+ ion and H0 atom impact, as would occur in a fusion reactor with a ratio [H+]/[H0] ~ 0.1 to 1, the total C-erosion yield is 2–5 times larger than the methane yield, depending on the ion energy and the relative fluxes. The absolute level of the maximum erosion yield under all conditions observed remains ≲ 0.1 C/H.

Hydrogen isotope retention in beryllium for tokamak plasma-facing applications

Beryllium has been used as a plasma-facing material to effect substantial improvements in plasma performance in the Joint European Torus (JET), and it is planned as a plasma-facing material for the first wall (FW) and other components of the International Thermonuclear Experimental Reactor (ITER). The interaction of hydrogenic ions, and charge-exchange neutral atoms from plasmas, with beryllium has been studied in recent years with widely varying interpretations of results. In this paper we review experimental data regarding hydrogenic atom inventories in experiments pertinent to tokamak applications and show that with some very plausible assumptions, the experimental data appear to exhibit rather predictable trends. A phenomenon observed in high ion-flux experiments is the saturation of the beryllium surface such that inventories of implanted particles become insensitive to increased flux and to continued implantation fluence. Methods for modeling retention and release of implanted hydrogen in beryllium are reviewed and an adaptation is suggested for modeling the saturation effects. The TMAP4 code used with these modifications has succeeded in simulating experimental data taken under saturation conditions where codes without this feature have not. That implementation also works well under more routine conditions where the conventional recombination-limited release model is applicable. Calculations of tritium inventory and permeation in the ITER FW during the basic performance phase (BPP) using both the conventional recombination model and the saturation effects assumptions show a difference of several orders of magnitude in both inventory and permeation rate to the coolant.

Model for the chemical erosion of graphite due to low-energy H+ and D+ impact

A methane erosion yield model has been developed using the principal atomistic reactions outlined by Kuppers and co-workers [eg., A. Horn, A. Schenk, J. Biener, B. Winter, C. Lutterloh, M. Wittmann, and J. Kuppers, Chem. Phys. Lett. 231, 193 (1994)] with additional terms to account for the energy of the incident particles, namely, kinetic ejection and damage deposition. Furthermore, modifications were made to the previous models by using distributed activation energies for methyl and hydrogen release as well as an activated Eley-Rideal abstraction process. Fitting of this model to experimentally measured methane yield data shows excellent agreement, except for low energy (⩽25 eV) impact at temperatures above ∼800 K. We have provided a sound physical basis for the behavior of the free fitting parameters and conclude that most of the processes associated with low-energy impact on pyrolytic graphite leading to methane production have been incorporated. Possible extensions of the model to include heavy hydroc...

Impurity release from low-Z materials under light particle bombardment

Abstract Recent progress in the understanding of the erosion of low-Z materials under bombardment conditions characteristic of magnetic fusion experiments is reviewed. The role of physical sputtering, chemical sputtering and radiation-enhanced sublimation in tokamaks is considered, and observations are related to laboratory measurements. The role of physical sputtering is largely understood, and tokamak measurements, under conditions where physical sputtering is expected to dominate, can be well predicted, except in the energy range near the sputtering threshold. Chemical erosion and radiation-enhanced sublimation are less-well understood, and predictions of erosion yields under tokamak conditions require assumptions (primarily related to energy and flux density dependence) which do not have a solid experimental basis. Also, only a few quantitative results from tokamaks are available to confirm predictions, and those which are available are not always consistent.

Flux and energy dependence of methane production from graphite due to H+ impact

Abstract Carbon is in widespread use for limiter surfaces, as well as first wall coatings in current tokamaks. Chemical erosion via methane formation, due to energetic H + impact, is expected to contribute to the total erosion rate of carbon from these surfaces. Experimental results are presented for the methane yield from pyrolytic graphite due to H + exposure, using a mass analyzed ion beam. H + energies of 0.1–3 keV and flux densities of ∼ 5 × 10 13 to 10 16 H + / cm 2 · s were used. The measured methane yield (CH 4 /H + ) initially increases with flux density, then reaches a maximum, which is followed by a sharp decrease. The magnitude of the maximum yield and the flux density at which it occurs depends on the graphite temperature. The yields obtained at temperatures corresponding to yield maxima at specific flux densities also show an initial increase, followed by a shallow maximum and a gradual decrease, as a function of flux density; the maximum occurs at ∼10 15 H + / cm 2 · s . Also presented are results on the methane production dependence on ion energy over the range 0.1 to 3 keV, and graphite temperature dependence measurements.

The removal of codeposited layers from TFTR tiles by O2 gas exposure

Graphite tiles from the TFTR inner bumper limiter have been exposed to O2 gas at 2100 Pa and 523–623 K in order to remove codeposited carbon/deuterium layers. It was found that most of the codeposited layer was removed at a rate ∼3 orders of magnitude faster than that observed for thinner laboratory plasma deposited films. Such fast erosion may make removal of codeposited films in ITER by O2 gas exposure a reasonable option.

Deuterium retention in beryllium, molybdenum and tungsten at high fluences

Abstract The retention of 1 keV D in room-temperature beryllium, molybdenum and tungsten has been measured as a function of fluence, for incident D3+ fluences ranging from 1021 to 1025 D/m2. The amount of D retained in Be tends to saturation (∼ 2.7 × 1021 D/m2) for incident fluences > 1022 D/m2, with an estimated saturation concentration (based on TRIM vectorized for multicomponent targets, TRVMC, range calculations) of ∼ 0.39 D/Be within the implantation zone; this retention level is similar to what has been observed for single-crystal graphite, viz, ∼ 2.2 × 1021 D/m2 or ∼ 0.4 D/C. For fluences greater than about 1022 D/m2, the amount of D retained in tungsten also tends to saturation at ∼ 6 × 1020 D/m2. Assuming a maximum ion implantation depth of ∼ 40 nm (as calculated by TRVMC), this amounts to a concentration of ∼ 0.24 D/W. The amount of D retained in Mo does not appear to reach saturation over the fluence range studied. D retention in Mo is lower than that in W for incident D fluences ≲ 1025 D/m2, at which point the two retention curves cross.